In internal combustion engines which have an arrangement for exhaust gas recirculation, the correct setting of an exhaust gas recirculation rate (AGR rate) is a key function for achieving the legally stipulated exhaust gas emissions. The AGR rate is equivalent to the volumetric percentage proportion of the exhaust gas in the fresh air-fuel mixture. The AGR rate is regulated, in this context, in a closed control loop, using a setting of the exhaust gas recirculation valve (AGR valve) in an exhaust gas recirculation system, which is positioned between an exhaust gas section for carrying off the combustion exhaust gas and an intake manifold for supplying an air/fuel mixture to the cylinders of the internal combustion engine, and by setting a throttle valve which determines the air supply into the intake manifold. The controlled variable may be the air mass or the AGR rate, for example.
Up to now, a fault in the exhaust gas recirculation system could be detected only by monitoring the system deviation. This is sufficient for detecting a defect in the actuators, such as the AGR valve and the throttle valve.
An additional possibility of a fault is that sooting may occur in the exhaust gas recirculation system, that is, a deposit of soot and oil vapors, which are able to reduce the flow cross section through lines of the exhaust gas recirculation system, or are able to impair the functioning of the AGR valve. However, sooting cannot be detected by monitoring the system deviation since, in the case of regulation, this is compensated for by greater opening of the AGR valve or by a more pronounced initial throttling of the fresh air mass by the throttle valve.
The more pronounced initial throttling of the supplied fresh air mass influences the control of the internal combustion engine, which may lead to a deterioration in the quality of exhaust gas emissions. Particularly in the case of operating products having a high exhaust gas recirculation rate, the air mass is then no longer able to be controlled by the setting of the AGR valve, the air mass being controlled in this case by a more pronounced initial throttling of the throttle valve.